In aqueous ink indirect printing, an aqueous ink is jetted on to an intermediate imaging surface, typically called a blanket, and the ink is partially dried on the blanket prior to transfixing the image to a media substrate, such as a sheet of paper. To ensure excellent print quality it is desirable that the ink drops jetted onto the blanket spread and be well-coalesced prior to drying. Otherwise, the ink images appear grainy and have deletions. Lack of spreading can also cause missing or failed inkjets in the printheads to produce streaks in the ink image. Spreading of aqueous ink is facilitated by materials having a high energy surface.
However, in order to facilitate transfer of the ink image from the blanket to the media substrate after the ink is dried on the intermediate imaging surface, a blanket having a surface with a relatively low surface energy is preferred. Rather than providing the desired spreading of ink, low surface energy materials tend to promote “beading” of individual ink drops on the image receiving surface.
Thus, an optimum blanket for an indirect image transfer process must tackle both the challenges of wet image quality, including desired spreading and coalescing of the wet ink; and the image transfer of the dried ink. The first challenge—wet image quality—prefers a high surface energy blanket that causes the aqueous ink to spread and wet the surface. The second challenge—image transfer—prefers a low surface energy blanket so that the ink, once partially dried, has minimal attraction to the blanket surface and can be transferred to the media substrate.
A third challenge, print-head management, must also be tackled. The third challenge relates to how well the print head carrying the ink jets can be kept clean of dried ink. For resin-based ink, the drying of the ink on the face plate of a print head can render it inoperable. On the other hand, too much moisture can condense on the face plate and cause jetting problems. In addition, some ink jets can be sensitive to high temperatures, typically temperatures above about 70° C.
Various approaches have been investigated to provide a solution that balances the above three challenges. These approaches include blanket material selection, ink design and auxiliary fluid methods. With respect to material selection, materials that are known to provide optimum release properties include the classes of silicone, fluorosilicone, a fluoropolymer, such as TEFLON or VITON, and certain hybrid materials. These materials have low surface energy, but provide poor wetting. Alternatively, polyurethane and polyimide have been used to improve wetting, but at the cost of poor ink release properties. Tuning ink compositions to address these challenges has proven to be very difficult since the primary performance attribute of the ink is the performance in the print head. For instance, if the ink surface tension is too high it will not jet properly and it if is too low it will drool out of the face plate of the print head. Compounding the problem is the fact that ink cohesion must be significantly greater than the ink-to-blanket adhesion for all image contents, including the stress cases of single layer small dot and three layer process black solid printing.
Polyurethane based topcoats are durable and are well known in the art. For example, polyurethanes are the basis of most automobile paints. Both aqueous based and non-aqueous based polyurethanes are known.
Thus far, the balance between the above discussed challenges has been elusive. Most solutions have tended to err toward optimizing image transfer from the blanket to the media substrate, with some sacrifice to image quality. What is needed is a low-cost solution to this problem that improves both wet image quality and image transfer without compromising the ink jet print head.